Direct Register Access For Host Simulation

ABSTRACT

Methods and apparatuses are provided that enable software designed to be operated with an embedded system to be tested in the absence of a physical embodiment of the embedded system. A simulation of the embedded system may be employed to operate the software. Various implementations of the invention provide for the processing of requests by the software to access memory within the embedded system. Still, various implementations of the invention provide for the identification of these memory access request and for the mapping of the desired memory location to a valid memory location.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/023,415 entitled “Implicit MemoryAccess Systems For Host Simulations,” filed on Jan. 24 2008, and namingAnwar Zeeshan et al. as inventors, which application is incorporatedentirely herein by reference.

FIELD OF THE INVENTION

The invention relates to the field of embedded system development. Morespecifically, various embodiments of the invention relate to memoryaccess in a host simulation environment.

BACKGROUND OF THE INVENTION Embedded Systems

In general, an embedded system may be described as a special purposecomputing system designed to perform one or a few dedicated functions.Embedded systems are commonly used in consumer devices like personaldigital assistants, mobile phones, videogame consoles, microwaves,washing machines, alarm systems, and digital cameras. In addition to theconsumer space, embedded systems are used in nearly every industry, fromtelecommunications to manufacturing, and from transportation to medicaldevices. In fact, embedded systems are so commonly in use today that itis not feasible to exhaustively list specific examples.

The term “embedded system” does not have a precise definition, anddetermining what is and is not an embedded system can be difficult. Forexample, a general purpose computer, such as a laptop, is not typicallycharacterized as an embedded system. However, a laptop is usuallycomposed of a multitude of subsystems such as the hard disk drive, themotherboard, the optical drive, the video processing unit, and variouscommunication devices. Many of the individual subsystems comprising thelaptop may themselves be embedded systems.

The complexity of embedded systems varies from those with a singlemicrocontroller chip and a light emitting diode to those with multiplemicroprocessor units and various peripheral communication interfaces andmechanical parts. Additionally, embedded systems range from those havingno user interface at all to those with full user interfaces similar to adesktop operating system. Manufacturers of modern microprocessors areincreasingly adding components and peripheral modules to theirmicroprocessors, creating what may be thought of as embedded processors.This type of embedded system is often referred to as a system on a chip(SoC). A simple example of a system on chip is an application-specificintegrated circuit (ASIC) packaged with a universal serial bus (USB)port.

There are many advantages to using embedded systems. For example,typically, an embedded system is designed to do some specific task, asopposed to being a general purpose computer with a wide range ofprogramming and performance features for performing many differenttasks. One benefit to using embedded systems as opposed to generalcomputing devices stems from the fact that embedded systems arededicated to specific tasks. As a result, design engineers can optimizethe embedded system for the desired task, which assists in reducing thesize and cost of the device as well as increasing its reliability andperformance. Furthermore, functionalities can be designed into anembedded system that would not be feasible using hardware alone. Byusing software to accomplish some of the functionality that would havebeen accomplished in hardware, designers may specify and define certainfunctionality later in the design cycle than was previously possible. Anadditional advantage is that embedded system designs can often bereconfigured for different functionality with less engineering overheadthan a design done strictly in hardware can. As a result, design reuseincreases and time to market is lowered.

The software written for embedded systems is generally referred to as“firmware.” Thus, the program used to control the hardware of anembedded system is referred to hereafter interchangeably as eithersoftware or firmware. Firmware is often stored on read only memory (ROM)based storage devices. For example, flash-based read only memory orelectronically erasable read only memory (EEPROM) devices are often usedto store firmware. The firmware controls the various features,functioning, and interfaces of the embedded system. Thus, a digitalvideo disk player will have firmware that processes the appropriateresponse to an input such as the user pressing the “power” button or the“play” button. Additionally, the firmware in this example would controlthe storage mechanism and the digital processing circuitry to decode andoutput onto the appropriate ports the video and audio signals stored onthe video storage medium.

A further advantage of embedded systems is the ability to update thefirmware, which provides longer term support and flexibility, a benefitoften not possible with purely hardware based systems. Updating anembedded systems firmware is often referred to as “flashing” the device.For example, manufacturers of most modern digital video disk playerstypically provide a means for the firmware stored on the device to beerased and a new version stored in place of the erased firmware. Asstated, a primary function of allowing the firmware to be updated is toprolong the usable life of the embedded system. For example, thefirmware of a digital video disk player could be updated to support newvideo encoding specifications. The new firmware file may be retrievedfrom an optical disk or other data storage medium, retrieved viaconnecting the device to a general purpose computer, or directly fromthe Internet via an Ethernet port.

Embedded System Development

Although packaging software and hardware together to create an embeddedsystem provides for great flexibility and increased functionality, italso complicates the design. The development of an embedded systemrequires knowledge of software and hardware design. Additionally, anembedded system development engineer will need to use tools to designand then test the electrical, mechanical, and software components of thedesign. Not only must the individual components be designed and tested,but the combination or interaction of the electrical, mechanical, andsoftware components must be tested together to ensure properfunctionality. Furthermore, complex devices require significantly morecomplex firmware to operate than do simple devices. As a result, theknowledge and skill required to design an embedded system and programits firmware increases as the complexity of the design increases.

The design of an embedded system is further complicated by the factthat, in many instances, the hardware and software are developedconcurrently. This prevents the verification engineers from using anactual embodiment of the designs hardware as the target with which totest the software on. In the absence of an actual embodiment of thehardware as a test target, special design and verification tools areneeded to test the software. One solution available to the embeddedsystem engineer is to test the software with a simulation of thehardware. The simulation could be a behavioral software simulation ofthe design, or it could be a hardware emulator configured to mimic thedesign. It is often possible with modern tools to simulate the hardwareand test the software with the same general purpose computing device(e.g., a computer workstation.) Using a simulation of the hardware asthe target with which to test the software component of the design isoften called host simulation. Embedded system developers frequentlyemploy host simulation due to the fact that in a host simulationenvironment, testing methods, as well as hardware and software designchanges, can be quickly developed and executed.

Employing host simulation in the test methodology may provide a decreasein design time and allow the hardware and software to be developed andtested concurrently. However, even using advanced host simulation tools,it is typically not possible to test the entirety of a software packageusing host simulation. Embedded software typically contains portionsreferred to as the “device layer” or “device driver.” These portions ofthe software are hardware dependent. More particularly, the device layerportions contain specific references to the actual hardware within thedesign, such as memory storage location addresses, such as the memorystorage location address associated with a particular data storageregister. Execution of device layer code in a host simulationenvironment will often trigger a memory access exception. Moreparticularly, when a specific instruction within the firmware specifiesthat data from a particular memory storage location address be read andthat particular address has not been explicitly allocated by the hostsimulation environment, a memory access exception will result.

This limitation in host simulation is commonly overcome by replacing thedevice driver sections of the software with direct function calls to thehardware simulator component of the host simulation environment.Although this method typically overcomes the limitation, it requires thedeveloper to maintain two copies of the software. More particularly, oneversion of the software must be maintained to test using host simulationand one version of the software that runs on the actual hardware must bemaintained. Maintaining two copies of the software creates extra workfor the developer and adds a significant amount of overhead.Additionally, maintaining two copies of the software provides moreopportunity and possibility to introduce errors into the software.

Another method of testing hardware-specific software in a hostsimulation environment is to allocate a memory array to simulate thememory-mapped hardware. The hardware memory accesses can be redirectedto this simulated memory array either by changing the hardware addressesin the code itself or by using linker options. One advantage toemploying a simulated memory array is that it allows for host simulationto be implemented while requiring that few, if any, changes be made tothe software. However, implementing host simulation with a simulatedmemory array also has some basic limitations. For example, when a memorymapped hardware element is accessed for a read or a write operation, theaction taken by the embedded systems hardware in response to this memoryaccess is often not a simple read or write operation. Therefore, asimulated memory array is insufficient to simulate the memory mappedtarget hardware and provide a suitable testing environment.

Still another method for dealing with memory access exceptions in hostsimulation testing is to catch the memory access exception that resultsfrom a hardware dependant memory access and redirect the memory accessinstructions to the hardware simulator component of the host simulationenvironment. Once the hardware simulator receives a memory accessinstruction, it calls the appropriate hardware simulator function toprocess the memory access instruction. Once the hardware simulator hasperformed its operation, the host simulation process may be resumed.This method is often referred to as implicit memory access (IMA).

Although implicit memory access provides a means to test hardwaredependant software, there are numerous limitations with implicit memoryaccess systems as well. First, it may not be possible with an implicitmemory access system to change the attributes of a memory accessinstruction. For example, Linux reserves the address range of0xC000_(—0000) to 0xFFFF_FFFF for use by the kernel. As a result, aninstruction seeking to access a memory storage location within thataddress range will produce a second memory access exception. Second,memory access systems cannot handle multiple threads at once. Forexample, the smallest memory page size in both the Linux and Windowsoperating systems is 4,000 bytes. Therefore, if multiple threads seek toaccess memory storage locations having addresses that lie within thesame memory page, a change in the attributes of the memory accessinstruction for one thread will interfere with the operation of theother threads. Third, any changes to the memory access instructionattributes by an implicit memory access system must be reverted,although this problem may be mitigated by various methods, such as usinga flag to halt execution of the host simulation environment, wherein thechanges may be reverted. However, some operating systems and softwaredebuggers used for performing host simulation do not allow particularflags to be used. Visual Studio debugger for example, reserves use ofthe processor Trap flag for setting breakpoints and its use by otherapplications interferes with the debugger. As a result, employing amethod involving a flag to revert any changes made to the memory accessinstructions attributes is not always possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described by way of illustrativeembodiments shown in the accompanying drawings in which like referencesdenote similar elements, and in which:

FIG. 1 illustrates a host simulation environment;

FIG. 2 illustrates an exemplary operating environment with which variousimplementations of the present invention may tied;

FIG. 3 illustrates an exemplary computing device with which variousimplementations of the present invention may be tied;

FIG. 4 illustrates a host simulation environment that may be implementedaccording to various embodiments of the present invention.

FIG. 5 illustrates a method that may be employed to practice variousimplementations of the present invention.

FIG. 6 illustrates a method that may be employed to practice variousimplementations of the present invention.

FIG. 7 illustrates a method that may be employed to practice variousimplementations of the present invention.

FIG. 8 illustrates an executable scratch memory array.

FIG. 9 illustrates a method that may be employed to practice variousimplementations of the present invention.

FIG. 10 illustrates a method that may be employed to practice variousimplementations of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Host Simulation

As introduced above, host simulation is the process of simulating ahardware device using software and employing the simulated version ofthe hardware device to test the firmware designed to run on the hardwaredevice. The hardware device is often referred to as the embedded systemor the target. FIG. 1 illustrates a host simulation environment 101. Ascan be seen in FIG. 1, the host simulation environment 101 includes ahost computing device 103. A host operating system 105 runs on the hostcomputing device 103. A target hardware simulator 107 may be run withthe host operating system 105. In various implementations of theinvention, the target hardware simulator 107 is included with the hostoperating system 105. As can be seen in FIG. 1, the target hardwaresimulator 107 is capable of instantiating a target hardware simulation109 of an embedded system 111. With various implementations of theinvention, the target hardware simulator 107 may be an independentprogram executed upon the host operating system 105. With still variousimplementations of the invention, the target hardware simulator 107 maybe an independent program executed upon a second computing device, whichcommunicates with the host computing device 103 and the host operatingsystem 105. The host operating system 105 is further capable ofexecuting or debugging an embedded software program 113. As can be seenin FIG. 1, the embedded software program 113 interacts with the hostoperating system 105, the target hardware simulator 107 and the targethardware simulation 109.

In various implementations of the invention, the target hardwaresimulator 107 may be an electronic design automation (EDA) tool, capableof simulating an embedded system. For example, the EDGE™ SimTest toolavailable from Mentor Graphics Corporation of Wilsonville, Oreg. may beused as the target hardware simulator 107. In some implementations ofthe invention, the host operating system 105 may be a Microsoft Windowsoperating system, such as for example, the Windows XP operating system.In further implementations of the invention, the host operating system105 may be a Unix based operating system, for example, Solaris™available from Sun Microsystems of Santa Clara, Calif. With stillfurther implementations of the invention, the host operating system 105may be a custom designed operating system, which may further include thetarget hardware simulator 107 and operate upon a general purposecomputing system.

As stated above, the target hardware simulator 107 is capable ofinstantiating a simulated version of the embedded system 111, such asthe target hardware simulation 109. In various implementations of theinvention, the embedded system 111 is in design form. More particularly,the embedded system 111 may be described by a specification detailingthe functioning and/or architecture of the embedded system 111. Variousfunctional description languages exist, such as for example the veryhigh speed integrated circuit hardware description language (VHDL,)which may be employed to describe the embedded system 111. Additionally,functional configurations may be specified, such as the memoryconfiguration and the input and output configuration.

The embedded software 113 can be a simple program for controlling theembedded system 111. For example, the embedded system 111 may be asimple spark timing controller for an automobile, with the embeddedsoftware 113 then being the software program that computes theappropriate spark timing based upon specified parameters. The embeddedsoftware 113 also may be a complex software program, including bothdevice driver code that interacts with the hardware of the embeddedsystem 111 and user interface code that specifies how a user mayinteract with the hardware of the embedded system 111. For example, theembedded software 113 may be a real time operating system (RTOS), suchas the Nucleus™ operating system available from Mentor GraphicsCorporation of Wilsonville, Oreg., or the Android™ operating systemavailable from Google of Mountain View, Calif.

Exemplary Operating Environment

As stated above, various electronic design automation tools or processesmay be implemented using computer executable software instructionsexecuted by one or more programmable computing devices. Because theseexamples of the invention may be implemented using softwareinstructions, the components and operation of a generic programmablecomputer system on which various embodiments of the invention may beemployed is described. Further, because of the complexity of someelectronic design automation processes and the large size of manycircuit designs, various electronic design automation tools areconfigured to operate on a computing system capable of simultaneouslyrunning multiple processing threads. The components and operation of acomputer network 201 having a host or master computer and one or moreremote or slave computers therefore will be described with reference toFIG. 2. This operating environment is only one example of a suitableoperating environment, however, and is not intended to suggest anylimitation as to the scope of use or functionality of the invention.

In FIG. 2, the computer network 201 includes a master computer 203. Inthe illustrated example, the master computer 203 is a multi-processorcomputer that includes a plurality of input and output devices 205 and amemory 207. The input and output devices 205 may include any device forreceiving input data from or providing output data to a user. The inputdevices may include, for example, a keyboard, microphone, scanner orpointing device for receiving input from a user. The output devices maythen include a display monitor, speaker, printer or tactile feedbackdevice. These devices and their connections are well known in the art,and thus will not be discussed at length here.

The memory 207 may similarly be implemented using any combination ofcomputer readable media that can be accessed by the master computer 203.The computer readable media may include, for example, microcircuitmemory devices such as random access memory (RAM), read-only memory(ROM), electronically erasable and programmable read-only memory(EEPROM) or flash memory microcircuit devices, CD-ROM disks, digitalvideo disks (DVD), or other optical storage devices. The computerreadable media may also include magnetic cassettes, magnetic tapes,magnetic disks or other magnetic storage devices, punched media,holographic storage devices, or any other medium that can be used tostore desired information.

As will be discussed in detail below, the master computer 203 runs asoftware application for performing one or more operations according tovarious examples of the invention. Accordingly, the memory 207 storessoftware instructions 209A that, when executed, will implement asoftware application for performing one or more operations. The memory207 also stores data 209B to be used with the software application. Inthe illustrated embodiment, the data 209B contains process data that thesoftware application uses to perform the operations, at least some ofwhich may be parallel.

The master computer 203 also includes a plurality of processor units 211and an interface device 213. The processor units 211 may be any type ofprocessor device that can be programmed to execute the softwareinstructions 209A, but will conventionally be a microprocessor device.For example, one or more of the processor units 211 may be acommercially generic programmable microprocessor, such as Intel®Pentium® or Xeon™ microprocessors, Advanced Micro Devices Athlon™microprocessors or Motorola 68K/Coldfire® microprocessors. Alternatelyor additionally, one or more of the processor units 211 may be a custommanufactured processor, such as a microprocessor designed to optimallyperform specific types of mathematical operations. The interface device213, the processor units 211, the memory 207 and the input/outputdevices 205 are connected together by a bus 215.

With some implementations of the invention, the master computing device203 may employ one or more processing units 211 having more than oneprocessor core. Accordingly, FIG. 3 illustrates an example of amulti-core processor unit 211 that may be employed with variousembodiments of the invention. As seen in this figure, the processor unit211 includes a plurality of processor cores 301. Each processor core 301includes a computing engine 303 and a memory cache 305. As known tothose of ordinary skill in the art, a computing engine contains logicdevices for performing various computing functions, such as fetchingsoftware instructions and then performing the actions specified in thefetched instructions. These actions may include, for example, adding,subtracting, multiplying, and comparing numbers, performing logicaloperations such as AND, OR, NOR and XOR, and retrieving data. Eachcomputing engine 303 may then use its corresponding memory cache 305 toquickly store and retrieve data and/or instructions for execution.

Each processor core 301 is connected to an interconnect 307. Theparticular construction of the interconnect 307 may vary depending uponthe architecture of the processor unit 301. With some processor cores301, such as the Cell microprocessor created by Sony Corporation,Toshiba Corporation and IBM Corporation, the interconnect 307 may beimplemented as an interconnect bus. With other processor cores 301,however, such as the Opteron™ and Athlon™ dual-core processors availablefrom Advanced Micro Devices of Sunnyvale, Calif., the interconnect 307may be implemented as a system request interface device. In any case,the processor cores 301 communicate through the interconnect 307 with aninput/output interfaces 309 and a memory controller 311. Theinput/output interface 309 provides a communication interface betweenthe processor unit 211 and the bus 215. Similarly, the memory controller311 controls the exchange of information between the processor unit 211and the system memory 207. With some implementations of the invention,the processor units 211 may include additional components, such as ahigh-level cache memory accessible shared by the processor cores 301.

While FIG. 3 shows one illustration of a processor unit 211 that may beemployed by some embodiments of the invention, it should be appreciatedthat this illustration is representative only, and is not intended to belimiting. For example, some embodiments of the invention may employ amaster computer 203 with one or more Cell processors. The Cell processoremploys multiple input/output interfaces 309 and multiple memorycontrollers 311. Also, the Cell processor has nine different processorcores 301 of different types. More particularly, it has six or moresynergistic processor elements (SPEs) and a power processor element(PPE). Each synergistic processor element has a vector-type computingengine 203 with 128×128 bit registers, four single-precision floatingpoint computational units, four integer computational units, and a 256KB local store memory that stores both instructions and data. The powerprocessor element then controls that tasks performed by the synergisticprocessor elements. Because of its configuration, the Cell processor canperform some mathematical operations, such as the calculation of fastFourier transforms (FFTs), at substantially higher speeds than manyconventional processors.

It also should be appreciated that, with some implementations, amulti-core processor unit 211 can be used in lieu of multiple, separateprocessor units 211. For example, rather than employing six separateprocessor units 211, an alternate implementation of the invention mayemploy a single processor unit 211 having six cores, two multi-coreprocessor units 211 each having three cores, a multi-core processor unit211 with four cores together with two separate single-core processorunits 211, or other desired configuration.

Returning now to FIG. 2, the interface device 213 allows the mastercomputer 203 to communicate with the slave computers 217A, 217B, 217C .. . 217 x through a communication interface. The communication interfacemay be any suitable type of interface including, for example, aconventional wired network connection or an optically transmissive wirednetwork connection. The communication interface may also be a wirelessconnection, such as a wireless optical connection, a radio frequencyconnection, an infrared connection, or even an acoustic connection. Theinterface device 213 translates data and control signals from the mastercomputer 203 and each of the slave computers 217 into network messagesaccording to one or more communication protocols, such as thetransmission control protocol (TCP), the user datagram protocol (UDP),and the Internet protocol (IP). These and other conventionalcommunication protocols are well known in the art, and thus will not bediscussed here in more detail.

Each slave computer 217 may include a memory 219, a processor unit 221,an interface device 223, and, optionally, one more input/output devices225 connected together by a system bus 227. As with the master computer203, the optional input/output devices 225 for the slave computers 217may include any conventional input or output devices, such as keyboards,pointing devices, microphones, display monitors, speakers, and printers.Similarly, the processor units 221 may be any type of conventional orcustom-manufactured programmable processor device. For example, one ormore of the processor units 221 may be commercially generic programmablemicroprocessors, such as Intel® Pentium® or Xeon™ microprocessors,Advanced Micro Devices Athlon™ microprocessors or Motorola 68K/Coldfire®microprocessors. Alternately, one or more of the processor units 221 maybe custom manufactured processors, such as microprocessors designed tooptimally perform specific types of mathematical operations. Stillfurther, one or more of the processor units 221 may have more than onecore, as described with reference to FIG. 3 above. For example, withsome implementations of the invention, one or more of the processorunits 221 may be a Cell processor. The memory 219 then may beimplemented using any combination of the computer readable mediadiscussed above. Like the interface device 213, the interface devices223 allow the slave computers 217 to communicate with the mastercomputer 203 over the communication interface.

In the illustrated example, the master computer 203 is a multi-processorunit computer with multiple processor units 211, while each slavecomputer 217 has a single processor unit 221. It should be noted,however, that alternate implementations of the invention may employ amaster computer having single processor unit 211. Further, one or moreof the slave computers 217 may have multiple processor units 221,depending upon their intended use, as previously discussed. Also, whileonly a single interface device 213 or 223 is illustrated for both themaster computer 203 and the slave computers 217, it should be notedthat, with alternate embodiments of the invention, either the mastercomputer 203, one or more of the slave computers 217, or somecombination of both may use two or more different interface devices 213or 223 for communicating over multiple communication interfaces.

With various examples of the invention, the master computer 203 may beconnected to one or more external data storage devices. These externaldata storage devices may be implemented using any combination ofcomputer readable media that can be accessed by the master computer 203.The computer readable media may include, for example, microcircuitmemory devices such as random access memory (RAM), read-only memory(ROM), electronically erasable and programmable read-only memory(EEPROM) or flash memory microcircuit devices, CD-ROM disks, digitalvideo disks (DVD), or other optical storage devices. The computerreadable media may also include magnetic cassettes, magnetic tapes,magnetic disks or other magnetic storage devices, punched media,holographic storage devices, or any other medium that can be used tostore desired information. According to some implementations of theinvention, one or more of the slave computers 217 may alternately oradditions be connected to one or more external data storage devices.Typically, these external data storage devices will include data storagedevices that also are connected to the master computer 203, but theyalso may be different from any data storage devices accessible by themaster computer 203.

It also should be appreciated that the description of the computernetwork illustrated in FIG. 2 and FIG. 3 is provided as an example onlyand is not intended to suggest any limitation as to the scope of use orfunctionality of alternate embodiments of the invention.

Direct Register Access

As stated above, software designed to be operated with an embeddedsystem may, in the absence of a physical embodiment of the embeddedsystem, still be tested. This is often referred to as host simulation.More particularly, as illustrated by FIG. 1, the target hardwaresimulation 109 may be instantiated by the target hardware simulator 107and employed to test the embedded software 113. This process enables thetesting of the embedded software 113, even in the absence of a physicalembodiment of the embedded system 111. However, as further describedabove, the embedded software 113 often contain portions referred to asdevice driver code. The device driver code typically seeks to interactdirectly with specific areas of the embedded system 111, such as forexample a memory storage location having a specific address. Directregister access provides direct access by the device driver code of theembedded software 113 to the memory locations of a simulated instance ofthe embedded system 111.

FIG. 4 illustrates a host simulation environment 401 that may beimplemented according to various embodiments of the present invention.The host simulation environment 401, similar to the host simulationenvironment 101 is operated upon a computing device 103 and includes ahost operating system 105. Additionally, a target hardware simulator107, which may be operated upon the host operating system 105, isincluded. As can be seen from FIG. 4, the target hardware simulator 107is capable of instantiating a target hardware simulation 109.Additionally, in various implementations, the target hardware simulator107 instantiates a simulated memory 403. In various implementations, thesimulated memory 403 corresponds to the memory storage locationsavailable within the embedded system 111. The host simulationenvironment 401 further includes a register handler 405 and an exceptionhandler 407.

As stated above, many embedded systems have custom or specific memorystorage locations or registers for storing data. Additionally, manymicroprocessors employed in embedded systems have specific addressingrequirements for accessing these registers. If the embedded software 113seeks to access a memory storage location by an address that the hostoperating system 105 has not previously allocated or initialized, then amemory access exception will result. Additionally, if the embeddedsoftware 113 seeks to access a memory storage location by an addressingconvention that is not properly recognized or is recognized as “illegal”by the host operating system 105, then a memory access exception willresult.

The memory address referenced by the embedded software 113, which causesthe memory access exception, is referred to herein interchangeably as aninvalid memory address, illegal memory address, or faulty instruction.Additionally, as used herein, any memory access exception caused by aninstruction within the embedded software 113 is referred to as a memoryaccess exception or an illegal memory access interchangeably.Additionally still, the instruction within the embedded software 113that causes the memory access exception will hereafter be referred to asthe faulty instruction. Furthermore, occasionally the distinctionbetween the types of memory access exceptions, such as a read memoryaccess exception or a write memory access exception will be made.However, this is not always the case and the methods and processesdescribed herein are equally applicable to any memory access exceptionin a host simulation environment.

FIG. 5 illustrates a method 501 that may be implemented according tovarious embodiments of the present invention. The method 501 may beimplemented to perform direct register access in a host simulationenvironment, such as for example the host simulation environment 401 ofFIG. 4. As can be seen in FIG. 5, the method 501 includes an operation503 for identifying an illegal memory access exception and an operation505 for identifying the memory address referenced by the memory accessexception. In various implementations of the invention, the operation503 and the operation 505 are performed simultaneously. For example, amemory access exception may be reported by the host operating system 105that identifies the exception and further identifies the address of thememory access exception.

The method 501 for performing direct memory access further includes anoperation 507 for “mapping” the memory address causing the memory accessexception to a valid memory address. After which the instruction withinthe embedded software 113 that caused the memory access exception isagain executed. Furthermore, as can be seen in FIG. 5, the memoryaddress that caused the memory access exception is then “unmapped” fromthe valid memory address by the operation 511. In variousimplementations of the invention, the operation 507 replaces the faultyaddress in the faulty instruction with a valid memory address. Forexample, the valid memory address may reference a temporary variable.With some implementations of the invention, the exception handler 407determines the faulty memory address and the faulty memory instruction.More particularly, the exception handler 407 may perform the operation503 and the operation 505, by for example, “catching” or identifying thememory access exception and the invalid memory address.

As stated, the operation 503 may identify a memory access exceptioncaused by the embedded software 113 seeking to read from an invalidmemory address. With some implementations of the invention, the method501 further includes an operation 513 for placing data in the memorystorage location referenced by the valid memory address, hereafterreferred to as the valid memory location. In various implementations ofthe invention, the operation 513 copies data from the simulated memory403 to the valid memory location. For example, data stored in the memorystorage location within the simulated memory 403 that corresponds to thefaulty address may be copied or transferred into the valid memorylocation. The operation 513 may be performed if for example, the faultyinstruction is a read instruction. In various implementations of theinvention, the register handler 405 may be employed to perform theoperation 513.

As further stated above, the operation 503 may identify a memory accessexception caused by the embedded software 113 seeking to write to aninvalid memory address. Accordingly, with various implementations of theinvention, the method 501 further includes an operation 515 for copyingor transferring data from the valid memory location into a memorylocation within the simulated memory 413. For example, data stored inthe valid memory location may be copied or transferred to the memorystorage location within the simulated memory 413 that corresponds to thefaulty address. The operation 515 may be performed if for example, thefaulty instruction is a write instruction. In various implementations ofthe invention, the register handler 409 may be employed to perform theoperation 515.

Breakpoint And Soft-Breakpoint Usage in Direct Register Access

As can be seen in FIG. 5, it is desirable that the operation 511 undoany changes to the faulty instruction. For example, if the faultyaddress in the faulty instruction was replaced with a valid memoryaddress, the valid memory address should subsequently be replaced withthe faulty address. More particularly, if a valid memory address is leftin the faulty instruction, when the faulty instruction is again executedby the embedded software 113, no memory access exception would result.This would create a ripple effect causing the host simulation system 401to not correctly implement the implicit memory access system and theembedded software 113 would not be correctly tested.

FIG. 6 illustrates a method 601 for performing direct memory access,which may be implemented according to various embodiments of the presentinvention. The method 601 may be employed to practice the operation 507of FIG. 5. As can be seen in FIG. 6, the method 601 includes anoperation 603 for replacing the faulty address with a valid memoryaddress. The method 601 additionally includes an operation 605 forinserting a breakpoint instruction after the faulty instruction. Invarious implementations of the invention, the operation 605 inserts thebreakpoint instruction into the instruction queue, sometimes referred toas the instruction buffer or the instruction pipeline, for the embeddedsystem under test. With other implementations of the invention, theoperation 605 replaces the instruction following the faulty instructionwith the breakpoint instruction.

In various implementations of the invention, the breakpoint instructionis a memory write instruction. The breakpoint instruction seeking towrite a known value to a known invalid memory address. For example, thebreakpoint instruction could write a known byte of data to the memoryaddress 0xFFFF_FFFF, which would result in a memory access exception. Inother implementations the breakpoint instruction sets the TRAP flag,often referred to as the interrupt flag. In various implementations ofthe invention, the exception handler 407 may be programmed to identifythe memory access exception generated from writing the known value tothe known invalid memory address as the intended breakpoint. In furtherimplementations of the invention, the exception handler 407 may beprogrammed to identify the setting of the TRAP flag as the intendedbreakpoint.

With various implementations of the invention, the operation 509 of FIG.5 resumes execution of the embedded software program 411. Accordingly,after the faulty instruction is executed, the breakpoint instructionwill be executed, generating an additional memory access exception.However, this memory access exception may be recognized by the exceptionhandler 407 as the intended breakpoint and the operation 511 may beperformed as illustrated in FIG. 5. FIG. 7 illustrates a method 701 thatmay be implemented in various embodiments of the present invention toperform the operation 511. As can be seen in FIG. 7, the method 701includes an operation 703 for removing the breakpoint instruction and anoperation 705 for replacing the valid memory address in the faultyinstruction with the faulty address.

Temporary Variable And Executable Scratch Memory Usage in DirectRegister Access

As stated above, a memory access exception typically results from acomputer program instruction seeking to access an invalid memorylocation, such as a memory location that has not previously beenallocated. As further stated above, the operation 507 may replaces thefaulty address in the faulty instruction with a valid memory address.Many embedded software programs may execute multiple instructions atonce, often referred to as multi-threaded applications. Moreparticularly, the faulty instruction, with the replaced memory address,may be simultaneously executed by multiple processing threads of theembedded software program 411.

FIG. 8 illustrates an executable scratch memory array 801 that may beimplemented according to various embodiments of the present invention toperform direct memory access on a multi-threaded embedded softwareprogram. As can be seen in FIG. 8, the executable scratch memory array801 may be partitioned or divided into separate executable memorylocations 803. In various implementations of the invention, theexecutable memory locations 803 have a size of 32 bytes. With otherimplementations of the invention, the executable memory locations 803have a size of 64 bytes. Still, with other implementations of theinvention, the executable memory locations 803 do not have apredetermined size. Accordingly, the executable memory locations may bedynamically sized and allocated.

FIG. 9 illustrates a method 901 that may be implemented according tovarious embodiments of the present invention to perform direct memoryaccess using the executable scratch memory array 801. As can be seen inFIG. 9, the method 901 includes an operation 903 for recording theaddress of an instruction. In various implementations of the invention,the recorded instruction is the instruction following the faultyinstruction. The method 901 additionally, includes an operation 905 forencoding the faulty instruction into an executable memory location. Forexample, one of the executable memory locations 803 of FIG. 8.Furthermore, the method 901 includes an operation 907 for placing abreakpoint instruction to be executed after the encoded instruction.With various implementations of the invention the operation 907 employsa method similar to that illustrated in FIGS. 5 and 6, for causing abreakpoint instruction to be executed after the faulty instruction. Themethod 901 additionally includes an operation 909 for placing theexecutable memory location address into the instruction pointer and anoperation 911 for resuming execution of the embedded software program.In various implementations of the invention, the operation 911 causesthe embedded software program to begin execution from the faultyinstruction. As mentioned above, the instruction pointer is often alsoreferred to as the program counter and typically indicates the mechanismwith which the computing device maintains its instructions to beexecuted queue.

The method 901 further includes an operation 913 for placing therecorded instruction address in the instruction pointer and for resumingexecution of the embedded software program from the instruction pointer.As can be seen in FIG. 9, the operation 913 and the operation 915 areperformed after the intended breakpoint is identified. In variousimplementations of the invention, the intended breakpoint functionssimilarly to that as described in reference to FIG. 5, FIG. 6, and FIG.7.

String Processing In Direct Register Access Systems

Instructions that process strings typically will have a component thatincrements the memory address in order to accommodate the string datatype. This is especially true for those embedded software programswritten in assembly language. Accordingly, a string instructioncontaining a faulty memory address, when replaced with a valid memoryaddress will increment the valid memory address as the instruction isexecuted. FIG. 10 illustrates a method 1001 that may be implementedaccording to various implementations of the present invention to handlestring processing instructions in a direct memory access environment.

As can be seen in FIG. 10, the method 1001 includes an operation 1003for recording a count register value, an operation 1005 for setting thecount register value equal to 1, an operation 1007 for processing theinstruction, and an operation 1009 for decrementing the count registervalue. In various implementations of the invention, the count registeris the computations processing unit's iteration register. As can be seenin FIG. 10, the operations 1005, 1007, and 1009 may be repeated a numberof times. For example, if the recorded counter register value isinitially nine (9), then the process may be repeated nine (9) times. Invarious implementations of the invention, the operation 1007 employs amethod such as the method 501 of FIG. 5 to perform direct memory access.

CONCLUSION

As stated above, software designed to be operated with an embeddedsystem may, in the absence of a physical embodiment of the embeddedsystem, still be tested. A simulation of the embedded system may beemployed to operate the software. Additionally, methods for processingrequests by the software to access memory within the embedded system areprovided.

Although certain devices and methods have been described above in termsof the illustrative embodiments, the person of ordinary skill in the artwill recognize that other embodiments, examples, substitutions,modification and alterations are possible. It is intended that thefollowing claims cover such other embodiments, examples, substitutions,modifications and alterations within the spirit and scope of the claims.

1. A method of handling a memory access exception in a host simulationenvironment, comprising: identifying a memory access exception, thememory access exception corresponding to a first memory address and afirst instruction; mapping the first memory address to a second memoryaddress; and resuming execution of the first instruction.
 2. The methodrecited in claim 1, further comprising: identifying a set of memoryaccess parameters, the set of memory access parameters comprising aninstruction location, causing a breakpoint to occur after theinstruction location; and unmapping the first memory address from thesecond memory address.
 3. The method recited in claim 2, unmapping thefirst memory address from the second memory address comprising:replacing the second memory address in the first instruction with thefirst memory address.
 4. The method of handling a memory accessexception in a host simulation environment recited in claim 3, furthercomprising: providing a temporary variable, the temporary variablecorresponding to the second memory address.
 5. The method recited inclaim 4, wherein the temporary variable is a select variable from anarray of temporary variables.
 6. The method recited in claim 5, whereinthe array of temporary variables is a select array of temporaryvariables from a plurality of arrays of temporary variables.
 7. Themethod recited in claim 3, causing a breakpoint to occur after theinstruction location comprising: inserting a second instruction afterthe instruction location, the second instruction including: writing asingle byte to a known illegal memory location, wherein a second memoryaccess exception is caused; and identifying the second memory accessexception as the breakpoint.
 8. The method recited in claim 7, furthercomprising: storing the second instruction to a scratch memory storagelocation; and causing the second instruction to be executed from thescratch memory storage location.
 9. The method recited in claim 7,further comprising: storing the first instruction to a first scratchmemory storage location; storing the second instruction to secondscratch memory storage location; causing the first instruction to beexecuted from the first scratch memory storage location; and causing thesecond instruction to be executed from the second scratch memory storagelocation.
 10. The method recited in claim 3, unmapping the first memoryaddress to the second memory address further comprising: copying atleast a portion of data from a memory storage location corresponding tothe second memory address into a memory storage location correspondingto the first memory address.
 11. The method recited in claim 1, mappingthe first memory address to a second memory address comprising:replacing the first memory address in the first instruction with thesecond memory address.
 12. The method recited in claim 11, mapping thefirst memory address to a second memory address further comprising:copying at least a portion of data into a memory storage locationcorresponding to the second memory address.
 13. The method recited inclaim 1, resuming execution of the first instruction comprising:identifying an iteration register, the iteration register storing afirst value; storing the first value in a memory storage location;altering the value stored in the iteration register; executing the firstinstruction; causing a breakpoint to occur after the first instructionhas executed; adjusting the first value; and setting the value stored inthe iteration register to the adjusted first value.
 14. The methodrecited in claim 1, identifying a memory access exception comprising:decoding the first instruction, the decoded first instruction includingthe first memory address.
 15. A computer program product for enabling acomputer to process a memory access exception in a host simulationenvironment comprising: software instruction for enabling the computerto perform predetermined operations; and one or more computer readablemedium bearing the software instructions; the predetermined operationsincluding: identifying a memory access exception, the memory accessexception corresponding to a first memory address and a firstinstruction, mapping the first memory address to a second memoryaddress, and resuming execution of the first instruction, whereby thecomputer processes the memory access exception.
 16. The computer programproduct recited in claim 15, the predetermined operation for mapping thefirst memory address to a second memory address comprising: replacingthe first memory address in the first instruction with the second memoryaddress.
 17. The computer program product recited in claim 16, thepredetermined operation for mapping the first memory address to a secondmemory address further comprising: copying at least a portion of datainto a memory storage location corresponding to the second memoryaddress.
 18. The computer program product recited in claim 16, thepredetermined operation for resuming execution of the first instructioncomprising: identifying an iteration register, the iteration registerstoring a first value; storing the first value in a memory storagelocation; altering the value stored in the iteration register; executingthe first instruction; causing a breakpoint to occur after the firstinstruction has executed; adjusting the first value; and setting thevalue stored in the iteration register to the adjusted first value. 19.The computer program product recited in claim 15, the set ofpredetermined operations further comprising: identifying a set of memoryaccess parameters, the set of memory access parameters comprising aninstruction location; causing a breakpoint to occur after theinstruction location; and unmapping the first memory address from thesecond memory address.
 20. The computer program product recited in claim19, the predetermined operation for unmapping the first memory addressfrom the second memory address comprising: replacing the second memoryaddress in the first instruction with the first memory address.
 21. Thecomputer program product recited in claim 19, the predeterminedoperation for unmapping the first memory address from the second memoryaddress further comprising: copying at least a portion of data from amemory storage location corresponding to the second memory address intoa memory storage location corresponding to the first memory address. 22.The computer program product recited in claim 19, the predeterminedoperation for causing a breakpoint to occur after the instructionlocation comprising: inserting a second instruction after theinstruction location, the second instruction including: writing a singlebyte to a known illegal memory location, wherein a second memory accessexception is caused; and identifying the second memory access exceptionas the breakpoint.
 23. The computer program product recited in claim 22,the set of predetermined operations further comprising: storing thesecond instruction to a scratch memory storage location; and causing thesecond instruction to be executed from the scratch memory storagelocation.
 24. The computer program product recited in claim 22, the setof predetermined operations further comprising: storing the firstinstruction to a first scratch memory storage location; storing thesecond instruction to a second scratch memory storage location; causingthe first instruction to be executed from the first scratch memorystorage location; and causing the second instruction to be executed fromthe second scratch memory storage location.
 25. The computer programproduct recited in claim 24, wherein: the first scratch memory storagelocation is a select storage location from an array of executablescratch memory storage locations; and the second scratch memory storagelocation is a select storage location from the array of executablescratch memory storage locations.
 26. The computer program productrecited in claim 23, wherein the scratch memory storage location is aselect storage location from an array of executable scratch memorystorage locations.
 27. The computer program product recited in claim 19,the set of predetermined operations further comprising: providing atemporary variable, the temporary variable corresponding to the secondmemory address.
 28. The computer program product recited in claim 27,wherein the temporary variable is a select variable from an array oftemporary variables.
 29. The one or more computer-readable media recitedin claim 28, wherein the array of temporary variables is a select arrayof temporary variables from a plurality of arrays of temporaryvariables.